The airplane in straight-and-level unaccelerated flight is acted on by four
forces—lift, the upward acting force; weight, or gravity, the downward
acting force; thrust, the forward acting force; and drag, the backward
acting, or retarding force of wind resistance.

Lift opposes gravity.

Thrust opposes drag.

Drag and weight are forces inherent in anything lifted from the earth and
moved through the air. Thrust and lift are artificially created forces used
to overcome the forces of nature and enable an airplane to fly. The
engine and propeller combination is designed to produce thrust to overcome
drag. The wing is designed to produce lift to overcome the weight
(or gravity).

In straight-and-level, unaccelerated flight, (Straight-and-level flight is coordinated
flight at a constant altitude and heading) lift equals weight and thrust
equals drag, though lift and weight will not equal thrust and drag. Any
inequality between lift and weight will result in the airplane entering a
climb or descent. Any inequality between thrust and drag while maintaining
straight-and-level flight will result in acceleration or deceleration until
the two forces become balanced.

Lift = Constant x V 2

Drag = Constant x V 2

Lift and drag vary as the square of the velocity. The velocity of the relative wind passing over the wing is determined by the airspeed of the airplane. This means that as an airplane doubles its speed, lift and drag will quadruple as long as the angle of attack remains constant.1